Polyethylene is one of the most popular plastic in the world. Polyethylene is used to make grocery bags, shampoo bottles, toys, pipes, containers, drums, and even body armors. For such a versatile material, it has a very simple structure, the simplest of all commercial polymers. Specifically, polyethylene contains a long chain alkane with different alkyl substitutes. The long chain alkane having other long chain alkyl branching is called low density polyethylene (LDPE). The long chain alkane containing many short chain alkyl branches is called linear low density polyethylene (LLDPE). High density polyethylene (HDPE) has very little alkyl branching, resulting in the polyethylene with high crystallinity and high density. Linear polyethylene is generally much stronger than branched polyethylene, but branched polyethylene is typically cheaper and easier to make and process.
Ziegler-Natta catalyst systems have been used to make a wide variety of ethylene polymers including HDPE and LLDPE with high polymerization activity. The most widely used Ziegler-Natta catalyst systems comprise titanium, magnesium, halogen and optionally electron donors. It is known that high activities are exhibited especially when a Ziegler-Natta catalyst component is obtained from a liquid magnesium halogen, a liquid titanium compound and an electron donor.
Ziegler-Natta catalysts have been continuously developed for making polyethylene with higher activity and various polymer properties. For example, U.S. Pat. No. 7,153,803 discloses the addition of an alkyl silicate and a monoester in a traditional titanium catalyst. The resulted solid titanium catalyst component has increased catalytic activity. Employing this solid titanium catalyst component, less hydrogen is used in producing polyethylene with lower molecular weight.
The inventors have tested a number of organic compounds as additives in the Ziegler-Natta catalyst syntheses and found that adding a specific family of organic compounds in the catalysts can produce polyethylene with a narrow molecular weight distribution and other distinct polymer properties.